“Amplifiers” are any devices, circuits or systems capable of converting an input signal into an output signal with an increased energy level. Amplifier circuits and systems are used in many different applications. Conventional mobile phones, for example, commonly incorporate power amplifier systems to amplify radio frequency (RF) signals that are transmitted from the phone to a base station or other receiver. Generally, amplifier systems include any number of transistors or other non-linear gain elements that are capable of increasing an electrical current or voltage representing the input signal. The ratio of the magnitude of the output to the magnitude of the input of an amplifier is commonly called the “gain” of the amplifier.
In many applications, such as many conventional Global System for Mobile communications (GSM) telephones, the RF input power of the amplifier is held relatively constant during operation. Changes in the output power are accomplished by adjusting the bias level of the transistors found in the amplifier to adjust the amplifier gain. This arrangement, while effective in many ways, creates a relatively high degree of design sensitivity as the amplifying element is “biased on” by the RF input power. In particular, this design can be difficult to control during power-up, since the sudden presence of input RF power can result in the output power “snapping on” in a relatively quick manner. In many amplifier circuits, this quick start-up can produce transient signals that can lead to undesirable effects. Moreover, inherent instability at low bias levels can create undesirable oscillations in the amplifier, which in turn can lead to non-compliance with design specifications or other undesirable effects.
Accordingly, it is desirable to provide amplifier systems and techniques that improve startup performance. Other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.